Di- and polyamines of the diphenylmethane series are important starting materials for the preparation of the corresponding isocyanates. These are in turn prepared in large amounts and in particular are used for the preparation of polyurethanes. The industrial preparation of these isocyanates has been described many times in the literature and is achieved in particular by reacting the corresponding amines with phosgene in a solvent. In the sense of the present invention di- and polyamines of the diphenylmethane series are understood to mean amines and mixtures of amines of the following type:

Here, n stands for a natural number≥2. Hereinafter, the compounds of this type in which n=2 shall be referred to as diamines of the diphenylmethane series or diaminodiphenylmethane (hereinafter MMDA). Compounds of this type in which n>2 shall be referred to within the scope of this invention as polyamines of the diphenylmethane series or polyphenylenepolymethylene polyamines (hereinafter PMDA). Mixtures of both types shall be referred to as di- and polyamines of the diphenylmethane series (hereinafter MDA). The corresponding isocyanates, which can be derived formally by replacing all NH2 groups by NCO groups from the compounds of formula (I) shall be accordingly referred to as diisocyanates of the diphenylmethane series (hereinafter MMDI), polyisocyanates of the diphenylmethane series or polyphenylenepolymethylene polyisocyanates (hereinafter PMDI) or di- and polyisocyanates of the diphenylmethane series (subsequently MDI). Here, both in the case of the amine and also in the case of the isocyanate, the polymer (n>2) is generally always present in the mixture with the difunctional compound (n=2), and therefore in practice only two compound types are relevant: the pure diamines or diisocyanates (MMDA or MMDI) on the one hand and the mixture of difunctional compound and the polymer (MDA or MDI) on the other hand. Within the scope of the present invention streams of the amines or isocyanates shall be referred to as “pure” difunctional compounds (MMDA or MMDI) if the mass fraction of the difunctional compounds is at least 95.0% in relation to the total mass of the stream in question.
MDA is usually prepared by acid catalyzed reaction of aniline with formaldehyde with subsequent neutralization and preparation of the reaction product. The continuous, discontinuous or semi-continuous preparation of di- and polyamines of the diphenylmethane series is described in numerous publications and patents (for example EP-A-31 423; EP 934 922 B1; EP-B-1 167 343; EP-A-1 403 242; EP-A-1 707 557; EP-A-1 813 597; EP-A-1 813 598; U.S. Pat. No. 5,310,769; DE-A-198 04 918; JP-A-2004026753). The MDA obtained as reaction product contains substantially MMDA (as a mixture of the three technically notable MMDA isomers 4,4′-diaminodiphenylmethane, 2,4′-diaminodiphenylmethane and 2,2′-diaminodiphenylmethane) and the corresponding higher-nuclear PMDA homologues and PMDA isomers. In addition, numerous secondary products and trace components are also contained in the MDA in very different fractions. In particular, crude MDA does routinely contain unreacted aniline and water. Their separation to less than 1000 ppm (aniline) or less than 200 ppm (water) by an at least two-step distillation process comprising a flash evaporation and subsequent cooling is described in EP-A-1 813 597.
The main use of MDA is the aforementioned preparation of the corresponding isocyanate. In most industrial processes the MDA is phosgenated directly into MDI; i.e. the MDA as obtained in the acid catalyzed condensation of aniline and formaldehyde (without separation into its isomers or homologues) is subjected to phosgenation. Only at the isocyanate stage does separation occur into the diisocyanate (MMDI) and a mixture of MMDI and PMDI, the MMDI fraction being reduced in relation to the crude product in accordance with the amount of the diisocyanate separated off.
For certain fields of application, for example as a crosslinker in plastics or varnishes, the amine itself may also be used, whether in the form of the aforedescribed mixture of isomers and homologues (MDA) or in the form of the diamine (MMDA). In addition, ring hydrogenation, in particular for the preparation of the ring-hydrogenated diamine (H12-MMDA, diaminodicyclohexylmethane), is known (see for example WO 2009/144148 A1, WO 2008/077672 A1). H12-MMDA for its part can be converted by means of phosgenation or via alternative processes into the corresponding isocyanate (H12-MMDI, dicyclohexylmethane diisocyanate) (see for example WO 2009/144148 A1, WO 2008/077672 A1). For many such fields of application not relating to the preparation of MDI, it is desirable to have the diamine (MMDA) available in the greatest purity possible, i.e. with the smallest possible fraction of polymers.
In addition, pure MMDA, in contrast to MDA (which in the terminology of the present invention always contains substantial fractions of higher homologues that can only be evaporated with difficulty) can be phosgenated without problem in the gas phase into MMDI, such that it is possible to make use of the advantages of gas-phase phosgenation known from the preparation of toluene diisocyanate (TDI).
A series of processes for the separation of MMDA from MDA are known from the prior art:
Thus, MMDA can be separated off and the pure state of the 4,4′-MMDA can be isolated by means of extraction, as described for example in SU 463 658, by means of reaction with metal salts, as described in GB 1 169 127, by melting, as described in EP-A-0 572 030, or by treatment with solvents, as described in BE 855 402 and U.S. Pat. No. 4,034,039.
RO 104327 B1 describes the separation of MMDA by means of thin-film distillation. It is also known to separate off MMDA from MDA by means of distillation.
DE-OS-1 901 993 describes a process for the preparation of 4,4′-MMDA, in which the MMDA is distilled off from a mixture of MMDA and PMDA and then 4,4′-MMDA is separated off by crystallization. The distillation is performed at 2 Torr and 220 to 230° C.
DE-OS-100 31 540 describes a process for separating off 2,2′-MMDA and 2,4′-MMDA from MDA. To this end, a distillation column having at least 40 separation stages can be used. The distillation is performed with a temperature profile of from 180 to 280° C., at a head pressure of from 0.1 to 10 mbar, and at a sump pressure of from 8 to 20 mbar. Fabric packing that has a low pressure loss is used in order to reduce the pressure losses. The MDA freed of 2,2′- and 2,4′-MMDA is reacted with phosgene to form MDI; the separated-off 2,2′- and 2,4′-MMDA is fed back into the condensation stage.
WO 2006/103189 A1 describes the preparation of MDA, separation of same into a partial stream containing substantially MMDA and a partial stream containing the remaining MMDA and PMDA, and separate phosgenation of the two partial streams. The partial stream containing substantially MMDA is phosgenated in the gas phase and the partial stream containing the remaining MMDA and PMDA is phosgenated in the liquid phase. The separation of the two amine partial streams is preferably performed with distillation, for example in two successive distillation columns or in a dividing wall column, wherein in the case of the latter embodiment the partial stream consisting substantially of MMDA is removed in a side drain of the dividing wall column.
Separation of 4,4′-MMDA by distillation is described in WO 2007/085534 A1, wherein the MDA is divided into two partial streams. The first partial stream is for example divided into a main stream containing substantially 4,4′-MMDA, a sump stream and a head stream in two successive columns or in a dividing wall column. The sump stream and head stream are combined again with the second partial stream and are used for example as starting material for phosgenation to form MDI.
A disadvantage of all previous processes for separating off MMDA and/or for the separation of the MMDA isomers is that a separate process requiring a complex equipment set-up is necessary (for example two-stage distillation or the use of a dividing wall column, which is difficult to control), in which the secondary products which are created usually have to be disposed of. In addition, the use for example of a separate distillation stage may lead to a reduction in the quality of the obtained MDA and the MDI prepared therefrom due to the increased thermal load.
There was thus a need for a process which makes it possible to recover MMDA from MDA with a high level of purity, i.e. in particular with a minimal fraction of PMDA, without having to exert a high process (distillation) effort. In particular, the process should be suitable for integration in the simplest manner possible into existing plants for the production of MDA, and, once the MMDA has been separated off, the remaining MDA (with a necessarily reduced MMDA fraction) should also be suitable for the usual uses of MDA.